Many cable television service providers use devices known as converter terminals to convert cable television signals into a radio frequency (RF) signal that can be used by conventional television equipment. A typical converter terminal receives cable television signals through an RF input. The cable television signals are then provided to a signal processor that includes a tuner, which can select and tune to one of the various channels included in the incoming cable television signal for display on a television.
A signal decoder receives the audio and video components of the signal for the selected or tuned channel. The decoder then processes the components of the tuned signal and provides output signals to such devices as a television or video cassette recorder (VCR) through one or more RF outputs.
As home theater systems have become more popular, the demand for high-fidelity stereo sound has increased. To meet this growing demand, some cable service providers have begun to offer CD-quality sound to their subscribers using a digital standard known as Near Instantaneously Companded Audio Multiplex (NICAM). These providers transmit composite signals that contain, in addition to the conventional audio and video signal components, a digital NICAM signal component. This NICAM signal component is decoded by the subscriber's television receiver and reproduced as high-fidelity stereo sound.
Some conventional converter terminals pass the NICAM signal component through their signal processing circuitry along with the standard audio and video components of the cable signal. FIG. 1 depicts an example of a typical converter terminal that handles NICAM signals in this way.
In FIG. 1, the converter terminal receives an RF signal carrying multiple channels of cable television programming through an RF input 100 terminal. A tuner 102 then selects and tunes to one of these channels and passes the tuned signal through a composite filter arrangement 104 to separate the tuned signal into audio and video signal components.
The composite filter arrangement 104 includes a channel surface acoustic wave (SAW) filter 106 that substantially filters out undesired frequencies from the tuned signal. As a result, the channel SAW filter 106 passes a composite signal that contains video, standard audio, and NICAM audio signal components. An intermediate frequency (IF) strip 108 amplifies the composite signal to compensate for attenuation caused by the channel SAW filter 106. The amplified signal is then further filtered by a dual SAW filter 110, which is made up of two distinct SAW filters and has two corresponding outputs: an audio output and a video output.
Accordingly, the composite filter arrangement 104 receives the tuned signal from the tuner 102 and separates its audio and video components. The bold line on FIG. 1 indicates that the composite filter arrangement 104 generates multiple outputs.
An audio/video amplifier 112 then amplifies the audio and video signal components to compensate for attenuation by the composite filter arrangement 104. Next, an audio/video demodulator 114 downconverts the amplified audio and video signal components to their respective baseband frequencies. It should be noted that FIG. 1 depicts only the audio output of the audio/video demodulator 114, as the video output is not of interest for the purposes of this discussion.
The audio output of the audio/video demodulator 114 contains both NICAM and standard audio signal components. A NICAM passive filter 116 passes the NICAM component, which is then upconverted to an RF frequency by an RF modulator 118 and provided to the television or other equipment along with the standard audio and video signal components using an RF output 120.
While this approach is commonly used to pass NICAM signals for reproduction as high-fidelity stereo sound, it suffers from certain limitations that adversely affect the NICAM signal, resulting in lower sound quality. As with all digital signals, the NICAM standard uses discrete signal levels to represent high and low logic values. In the passthrough approach of FIG. 1, however, these signal levels are made somewhat less distinct, leading to potential confusion of the logic values. The subscriber hears these signal defects as audio artifacts, such as popping sounds.
One source of NICAM signal degradation is the NICAM passive filter 116, which typically has more than 300 nanoseconds of propagation delay and lacks sufficient rejection or attenuation within 50 KHz of the audio carrier frequency to remove all the non-NICAM signal components to obtain only the NICAM signal component. Furthermore, the NICAM passive filter 116 typically requires multiple alignments, or adjustments, to perform the required filtering. This alignment process is time consuming, often taking minutes to complete, and is susceptible to human error.
Even if all of the alignments are performed correctly, temperature fluctuations in the operating environment and component aging alter the frequency response of the NICAM passive filter 116. As a result, the passband or notch of the NICAM passive filter 116 drifts from the original desired state to a frequency other than the desired frequency.
All of these limitations impair the ability of the NICAM passive filter 116 to separate the NICAM and non-NICAM signal components. This difficulty is further compounded by the relative closeness in frequency of the NICAM audio, standard audio, and video signal carriers. With other signal components substantially present in the NICAM signal component, a phenomenon known in the industry as eye height degradation occurs, in which the distinction between high and low signal levels is blurred.
The NICAM signal component is subject to further degradation from other causes. For example, it is difficult to design analog filters having the steep roll off characteristic required to preserve the near-instantaneous transitions between the logic high and logic low levels. The less-steep roll off that more typically characterizes analog filters causes the logic level transitions to be less instantaneous, leading to further confusion between logic levels.